Edexcel IGCSE ICT
Processors: The CPU
Big Idea: The CPU is the "brain" of the computer — it repeatedly fetches, decodes, and executes instructions, billions of times per second, and the faster it does this (and the more cores it has doing it at once), the more powerful the computer feels.
Quick SummaryEverything on this page, in one glance
- The CPU's job is to execute instructions, and it does this by repeating processor cycles — billions per second on a modern chip.
- Every processor cycle has three stages, always in this order: Fetch → Decode → Execute.
- Fetch: grab the next instruction/data from RAM and bring it to the CPU.
- Decode: figure out what that instruction actually means/needs.
- Execute: actually carry it out (calculate, store to RAM, or fetch more data).
- Processor speed is measured as clock speed, in Hertz (Hz) — this tells you how many cycles happen per second.
- Modern CPUs run in Gigahertz (GHz) — billions of cycles per second.
- A core is basically its own mini-CPU. More cores = more instructions handled in parallel.
- Total instructions/sec ≈ number of cores × clock speed (a simplification, but the exam logic you need).
1. What Is the Purpose of the CPU?Why does the computer even need one?
The CPU's One Job
Strip away all the jargon and the CPU has exactly one job: execute instructions. Every single thing your computer does — opening an app, moving your mouse cursor, rendering a game frame, playing a sound — eventually boils down to millions of tiny instructions that the CPU works through, one after another.
It does this by constantly repeating something called a processor cycle (sometimes called the fetch-decode-execute cycle, or FDE cycle). Think of a processor cycle as "one unit of work" — the CPU grabs one instruction, figures out what it means, does it, and then immediately starts again on the next one.
Analogy: Imagine a chef in a kitchen who can only do one thing at a time, but does it insanely fast. A recipe card comes in (fetch), the chef reads what it's asking for (decode), and then does that exact task — chop, stir, plate (execute). Then the next card comes in. A modern CPU is a chef who can do this billions of times every second.
Key Fact
A modern CPU can perform billions of processor cycles every second.
2. The Processor Cycle StagesFetch → Decode → Execute, in detail
Stage 1: Fetch
During the fetch stage, the CPU goes and grabs the next instruction or piece of data that it needs to work on. Where does it grab it from? RAM (the computer's main memory) — not the hard drive, and not storage. RAM holds whatever the CPU is actively using right now, precisely because it's fast to access.
Once fetched, that instruction/data is brought back to the CPU itself so it can be worked on in the next stage.
Why RAM and not the hard drive?
RAM is dramatically faster to read from than a hard drive or SSD. If the CPU had to fetch instructions from storage every cycle, it would be sitting idle most of the time waiting — like a chef waiting for ingredients to be delivered from a warehouse across town instead of grabbing them from the counter right next to them.
Stage 2: Decode
what the instruction is actually asking it to do
Stage 3: Execute
carries out
fetch